92 



LIFE BY THE SEASHORE. 



burrowing habit, for it enables the body fluid to move freely, 

 and, as we have seen, that has an important bearing in 

 relation to the method of movement. The absence of septa 



is further correlated with 

 the power of distending the 

 anterior part of the body, 

 which has an important me- 

 chanical effect in burrowing. 

 Running down the centre 

 of the body is the alimentary 

 canal (al in Fig. 31), which 

 in most bristle - worms per- 

 forates the septa, but which 

 is here almost free in the 

 body- cavity. Not entirely, 

 however, for in the anterior 

 region there are three sup- 

 porting mesenteries (s' } s", 

 s'"), and behind the gill- 

 bearing region there are many 

 of these. The anterior mesen- 

 tery, or diaphragm, is a struc- 

 ture of great interest. It is 

 completely circular, and is 

 attached to the alimentary 

 canal at the point where the 

 protrusible region, or buccal 

 cavity, opens into the next 

 region. During the burrow- 

 ing movements the body fluid 

 sweeps forward until it is 

 stopped anteriorly by this 

 circular diaphragm. It then 

 exerts a pressure upon the 

 diaphragm to which the latter 

 can yield in one way only 

 by the protrusion of the in- 

 trovert, which is doubled 

 FIG. 31. -Dissection of lob-worm from dor- outwards by the pressure of 



sal surface. For explanation, see text, the fluid behind. When the 

 In part after Gamble and Ash worth. , 



wave sweeps backwards again 



al 



